1. Field of the Invention
The invention relates to a component of an optical circuit which is based on an optical waveguide used in optical communication, an optical sensor or the like, and also to a method of producing the component.
2. Related Art of the Invention
In the fields of optical communication and an optical sensor, research and development of an optical circuit having various functions are vigorously conducted in order to realize sophisticated optical signal processing or sensing. An optical circuit uses an optical waveguide through which light passes, as a fundamental element. In an optical waveguide, a core region having a higher refractive index is disposed in a clad having a low refractive index, thereby confining light into the core region so as to propagate therethrough. When a core is patterned, various functions can be realized. In the specification, a configuration in which an optical waveguide is patterned in a manner like an electric circuit is defined as an optical circuit.
Hereinafter, an example of an optical circuit used in optical communication will be described in detail.
FIG. 10 is a schematic section view of a usual quartz single-mode optical waveguide. A core 101 has a square section shape having a side of 8 .mu.m, and is covered by a quartz clad 102. Light propagates in the direction of the arrow X.
In FIG. 11, (a) to (c) show a method of producing an optical waveguide which is used most commonly in the prior art (for example, Kawachi, "Optoelectronics" No. 8, p. 85, 1988). The production process includes the following steps.
(a) A core film 112 made of SiO.sub.2 doped with, for example, Ge is formed on the surface of a quartz substrate 111 serving also as a lower clad layer, by the flame deposition method (FIG. 11(a)). When a substrate other than a quartz substrate is used, a lower clad layer is previously formed on the substrate by the flame deposition method, and the core film 112 is then formed on the layer. PA0 (b) The core film 112 is patterned into a predetermined pattern by using the photolithography or dry etching technique, thereby forming a core portion 112a (FIG. 11(b)). PA0 (c) Finally, an upper clad layer 113 is formed by the flame deposition method to cover the core portion 112a (FIG. 11(c)).
According to this method, an optical waveguide of a low loss can be produced so that a complicated optical circuit is realized. Also methods in which the CVD method or the vapor deposition method is used as a film deposition method are under investigation.
Recently, such an optical waveguide device is more strongly requested to be produced at a low cost and in mass production.
In the field of optical communication, as typically exemplified by FTTH (Fiber To The Home), an optical fiber is being extended from a trunk line to a subscriber's line. Therefore, it is required to produce a photoelectric conversion module which is based on an optical waveguide, in mass and at a low cost.
The prior art method of producing an optical waveguide such as shown in FIGS. 11(a) to 11(c) has an advantage that even a complicated optical circuit of high performance can be produced. However, a long tact time is necessary for the film forming and heating processes because the core and the clad are produced by a thin film forming process. Furthermore, the photolithography or dry etching technique which is used in the patterning of the core requires many complicated steps. Consequently, although the method is suitable for production of an optical circuit of high performance and high added value, such as an array waveguide grating, it cannot say that the method is suitable for production of a simple optical circuit such as a Y-branch splitter.
In order to solve the problems in production of an optical waveguide, various ideas have been proposed. For example, one of potential processes is press molding. Press molding is proposed in Japanese Patent Publication (Kokai) Nos. HEI8-320420 and 1-26806, etc. As shown in FIGS. 12(a) to 12(c), a desired core pattern 120a is formed in a die 120, and the die is pressed against a base material (a first glass substrate 121) serving also as a lower clad at a high temperature (FIG. 12(a)). The first glass substrate 121 is taken out from the die 120, an ultraviolet curing resin 123 is applied to the surface and filled into a core portion 123a (FIG. 12(b)), and a second glass substrate 122 is stuck to the first glass substrate and the resin is then irradiated with ultraviolet rays to be cured. Thus core pattern grooves are formed in one operation (FIG. 12(c)). According to this method, a mass production is enabled, and the photolithography or dry etching step which is used in the prior art can be omitted, and a core can be easily formed by filling a resin. Therefore, this is seemed to be a hopeful process of producing an optical waveguide.
According to study of the inventors, however, it has been proved that the method of FIGS. 12(a) to 12(c) has the following drawbacks. When a glass material is subjected to press molding by using a die having a core pattern of a square section which has a side of about 8 .mu.m, large concentrated stress is applied to the core pattern portion because the aspect ratio of the vertical and lateral sides of the section of the core groove is large. Consequently, the die is easily broken, so that the die must be frequently replaced with a new one. When a glass material having a coefficient of thermal expansion which is largely different from that of a die is subjected to press molding, large thermal stress acts so as to tighten the pattern in the die, thereby lowering the release property between the die and the glass material. As a result, it is difficult to realize a satisfactory pattern transfer.
Furthermore, the production of a die used in press molding has the following problem.
Conventionally, a glass lens is known as an example of an optical component which is produced by using press molding. In a die for forming a glass lens, a hard alloy such as WC is mainly used as the base material, and a desired surface pattern is obtained by means of machining such as cutting or polishing.
As described above, a core of an optical waveguide has a rectangular section shape having a side of about 8 .mu.m. It is very difficult to produce such a shape by means of machining, because a cutting tool must usually have a radius diameter of 10 .mu.m or more. In machining, a linear core pattern can be formed, but it is very difficult to realize a branched pattern or a smoothly bent pattern.
On the other hand, etching is known as a production method other than machining. In etching, a complicated core pattern can be obtained, but, usually, a material of high mechanical strength is hardly etched. A hard alloy is not an exception to the above. When etching which is deeper than 5 .mu.m is to be performed, an etching mask must have a very complicated configuration. Furthermore, there is another problem in that the surface is roughened as a result of etching, and, even when the surface is smoothed by polishing, an edge of a section of the pattern is rounded, with a result that an optical waveguide of high performance can be hardly obtained.